Micro-electrical-mechanical system (MEMS) devices come in a variety of types and are utilized across a broad range of applications. One type of MEMS device that may be used in applications such as radio frequency (RF) circuitry is a MEMS vibrating device (also known as a resonator). A MEMS resonator generally includes a vibrating body in which a piezoelectric layer is in contact with one or more conductive layers. Piezoelectric materials acquire a charge when compressed, twisted, or distorted. This property provides a transducer effect between electrical and mechanical oscillations or vibrations. In a MEMS resonator, an acoustic wave may be excited in a piezoelectric layer in the presence of an alternating electric signal, or propagation of an elastic wave in a piezoelectric material may lead to generation of an electrical signal. Changes in the electrical characteristics of the piezoelectric layer may be utilized by circuitry connected to a MEMS resonator device to perform one or more functions.
Guided wave resonators include MEMS resonator devices in which an acoustic wave is confined in part of a structure, such as in the piezoelectric layer. Confinement may be provided by reflection at a solid/air interface, or by way of an acoustic mirror (e.g., a stack of layers referred to as a Bragg mirror) capable of reflecting acoustic waves. Such confinement may significantly reduce or avoid dissipation of acoustic radiation in a substrate or other carrier structure.
Various types of MEMS resonator devices are known, including devices incorporating interdigital transducer (IDT) electrodes and periodically poled transducers (PPTs) for lateral excitation. Examples of such devices are disclosed in U.S. Pat. Nos. 7,586,239, 7,898,158, and 8,035,280 assigned to RF Micro Devices (Greensboro, N.C., USA), wherein the contents of the foregoing patents are hereby incorporated by reference herein. Devices of these types are structurally similar to film bulk acoustic resonator (FBAR) devices, in that they each embody a suspended piezoelectric membrane. Such devices (including IDT-type devices in particular) are subject to limitations of finger resistivity and power handling due to poor thermal conduction in the structures. Additionally, IDT-type and PPT-type membrane devices may require stringent encapsulation, such as hermetic packaging with a near-vacuum environment.
Plate wave (also known as lamb wave) resonator devices are also known, such as described in U.S. Patent Application Publication No. 2010-0327995 A1 to Reinhardt et al. (“Reinhardt”). Compared to surface acoustic wave (SAW) devices, plate wave resonators may be fabricated atop silicon or other substrates and may be more easily integrated into radio frequency circuits. Reinhardt discloses a multi-frequency plate wave type resonator device including a silicon substrate, a stack of deposited layers (e.g., SiOC, SiN, SiO2, and Mo) constituting a Bragg mirror, a deposited AlN piezoelectric layer, and a SiN passivation layer. According to Reinhardt, at least one resonator includes a differentiation layer arranged to modify the coupling coefficient of the resonator so as to have a determined useful bandwidth. One limitation of Reinhardt's teaching is that deposition of AlN piezoelectric material (e.g., via epitaxy) over an underlying material having a very different lattice structure generally precludes formation of single crystal material; instead, lower quality material with some deviation from perfect orientation is typically produced. A further limitation is that Reinhardt's approach does not appear to be capable of producing resonators of widely different (e.g., octave difference) frequencies on a single substrate. Additionally, in at least certain contexts, it may be cumbersome to produce Bragg mirrors with consistently high reproducibility of layer thicknesses.
Accordingly, there is a need for guided wave devices that can be efficiently manufactured. Desirable devices would address thermal conduction and stringent packaging concerns associated with membrane-type devices. There is a further need to provide devices that may incorporate high quality piezoelectric materials. There is a still further need for devices that may enable production of widely different frequencies on a single substrate.